Transforming Growth Factor-? (TGF?) controls tissue homeostasis and orchestrates the response to tissue injury and repair. Cancers escape from TGF? 's homeostatic function but can activate the tissue repair function to enhance their invasive/metastatic phenotype. This concept has led to the development of TGF? pathway antagonists for cancer treatment. However, the absence of biomarkers for TGF? pathway addiction is a major roadblock in the development of these agents for clinical use. Our central hypothesis is that TGF? receptor gene mutations are indicative of pathway addiction and, therefore, predictive of response to antagonists. Over 40 cancer-associated missense mutations of the types I or -II TGF? receptor genes (TGFBR1, TGFBR2) have been identified. We have recently found that some of these constitutively activate the T?R-II receptor kinase, associated with loss of Smad2/3 activation on the one hand, and with de novo activation of Smad1/5 and a highly motile and invasive phenotype on the other. Moreover, treatment with selective T?R-ll kinase inhibitors reversed the transformed phenotype. In parallel, Marfan syndrome-like genetic disorders have recently been attributed to mutations of TGFBR1 or TGFBR2. These mutations also confer an activated phenotype on patient fibroblasts and vascular smooth muscle cells. Moreover, striking parallels exist between cancer- and Marfan-like syndrome-associated TGFBR mutations: (1) TGFBR2 gene mutations are much more common than TGFBR1 mutations; (2) TGFBR2 mutations are clustered in the C-lobe of the T?R-II kinase; and (3) many of the TGFBR2 mutated in human cancers coincide with those involved in Marfan-like syndromes as well as with the TGFBR2 orthologue in D. melanogaster. Thus, converging experimental evidence from these three very different sources strongly supports the hypothesis that TGF? receptor gene mutations can confer an aberrant activated cellular phenotype. Our specific aims are: 1. To predict the effects of TGFBR2 gene mutations on interactions with other proteins using structural biology approaches; 2. To determine which TGFBR2 gene mutations confer gain-of- function phenotypes using genetic assays in D. melanogaster; 3. To determine the effects of TGFBR2 gene mutations on TGF? signaling in mammalian cells; 4. To utilize selective chemical T?R kinase inhibitors to determine their potential therapeutic effects on gain-of-function TGFBR2 mutant cells. These studies are particularly important because TGF? antagonists are in (pre)clinical development for the treatment of (metastatic) cancer as well as fibrotic disorders. Thus, not only will our studies provide a precise understanding of the molecular pathobiology of disease-causing TGFBR mutations, but they will also inform the clinical development of TGF? pathway inhibitors for cancer and other disorders.